ligotky



Sept. 10, 1963 H. K. LIGOTKY 3,103,558

TONE DETECTOR Vous our INVENTOR. FREQUENCY H. K. L/GOTKY H. K. LlGoTKY3,103,558

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TONE DETECTOR H. K- LIGOTKY TONE DETECTOR Sept. 10, 1963 4 Sheets-Sheet4 Filed Sept. 24, 1959 Oli United States Patent O 3,103,558 TONEDETECTOR" Harri Kurt Ligotly, Chicago, Ill., assignor to InternationalTelephone & Telegraph Corporation, New York, NY., a corporation ofMaryland Filed Sept. 24, 1959, Ser. No. 842,136 11 Claims. (Cl.179-84.).

This invention relates to tone detectors and more particularly to meansfor separating signals which lie in the same frequency spectrum.

Although harmonics ofthe human voice may include frequencies which arehigher than 3000 cycles per second', it has been found that such higherfrequencies may be eliminated without loss of intelligence. Therefore,in the past typical telephone equipment has been designed to limit voicechannels to a bandwidth of about 01 to 30010*y cycles per second.However, it is not enoughvmerely to provide for the transmission ofvoice signals, it is also necessary to provide for transmitting controlsignals which may be used to establish and maintain connections througha telephone system. For example, it is necessary to transmit digitpulses, off-hook supervision, and `the like. IIf an extra con-trolchannel is provided, thereis a wasteful duplication of parts. If .aband-width of more than' 3000 cycles per second is provided for eachaudio channel, there is a needless -waste of communication facilities.Therefore, it is customary to use controlv signals having frequencieswhich may be passed through the equipment that is also used to conveyspeech signals. The problem is to separate the speech and controlsignals sov that each may properly perform its functions.

In the past, it has been common practice to separate Voice and controlsignals according to the frequency thereof. That is, sharply tunedband-pass iilters pickout the frequencies used -for control signals.Among other things, the disadvantages of such prior systems are thatsharply tuned, band-pass filters are lVery expensive and that voicetransmission is 'degraded' by an elimination of the band of frequencieswhich are utilized to transmit control signals. An attemptto improve thequality of voice transmission by reducing the bandwidth of thesignalchannel results in higher cost iilters. If an effort is made toreduce the cost of the iilters, a broader band mustbe usednfor.` the.transmission of'controlf signals, thereby `further degrading the voicesignals. Moreover, means must be providedto accommodate a widevfluctuation of signal strength between the tone components ofcompound-tone controlsignals, the length of. control signal' pulses mayvary yfrom a desired standard, andv occasionally the voice frequenciesmay exactly simulate' control signals. p Y

Anobject of this invention' is to provide new and improved means forseparating two types of signals which lie in the same acoustic spectrum.

Another object of this invention is to provide means for separatingfdiierent types of signals without requiring expensive, sharply tuned,band-pass lt'ers.

Yet another'object of this invention is to provide means' for separatingvoice and control signals, wherein signal strength may diuctuate .over awide range'.

Still another object of thisinvention is to provide' 'meansfordetectingV individual tone components which mayA hav a wide variance ofsignal strength. i

A further object ofthis invention isy to' provide`` a tonedetectorhaving immunity to voice' signals.

This invention provides means for. separating voice" and control signalswhich fall within a single acoustic spectrum. The separation islaccomplished through thev interaction of an ampliiier used as a'signalsuppressor 3,103,558 Patented Sept. 10, 1963 "lee and a constant outputamplifier which cooperate to change the relative strength of voice andcontrol signals. 'Ihat is, ea'ch -voice channel or transmission mediumconveys electrical signals representing a single band which includesboth voice and control signals. A sample of all such electrical signalsis bled-off and fed through a signal suppressor which distorts thesignal'by greatly amplifying an exclusively voice signal portion of theband but which does not aiect the control portion of the band. Thedistorted signal is then fedv through a constant output amplifier, i.e.an amplifier automatic gain control. Therefore, if present, thebroadband of greatly amplied voice signals swamps succeeding equipmentand the narrow band of unamplied control signals is too weak to have anypractical elfect. On the other hand, if broad band voice signals are notpresent, there is little effect in the signal suppressor; substantiallythe entire output of the constant output amplifier consists of thenarrow band of control signals, andy succeeding. equipment respondsthereto.

The above mentionedand other objects of. this invention together withthe manner of obtaining them will become more apparent and the inventionitself will be best understood by making reference to the. followingdescription of the invention taken in conjunction with the accompanying'drawings in which:

FIG. 1A shows prior art;

FIG. 1B shows a characteristic spectrum of av telephone transmitter andof voice transmission in the subject invention;

EIG. 1C shows a characteristic spectrum ofl prior art voice and controlsignals;

lFIG. lD'shows how the subject invention connects into a voice channel;l

FIG. 2 illustrates a tone receiver by' means of'a block diagram;

FIG. 3A illustrates the characteristics ofthe suppressor;

FIG. 3B illustrates how suppressor 11 and amplifier 12 cooperate toydistort the signal;

FIG. 4 shows the cincuitry of suppressor l1'\1;

PIG'. 5 shows circuit details of one channel of the tone detector; and lFIG. 6 illustrates the' characteristics of the various items which areshown in FIG. 5.V

Where possible, simple terms are used and specic items are describedhereinafter to' facilitatev an understanding of theinvention; however,it should' be understood that theuse of suchY terms andlreferences tosuch items are'- not to act in any manner as a :disclaimer of the full'range of equivalents which are normally' given under es# tablishedrulesof patent law. For example, hereinafter" the specificationv refers todetection of control signals which are transmitted asl tones that'. are'included inthe acoustical spectrum of the human voice; whereas', thein'- vention has utility any timethatcsignals in the.' same spectrum areto be separated. `More'overgtheV drawings' show transistorized circuits;Whereas,\. electron/cubes or otherl suitable components mayr be used".drawings show speci-lite. details of ampi-'ittica circuits; whereas, anysuitable amplifierv and filter combinationsl may-be. used. Quiteobviously, other examples could be'E selected to illustrate the mannerinwhich' the terms that? have been used'and the items which have'beendscribed are entitled toA awide range of equivalents:`

v Brief DescrprcmY Irl-typical prior art systems, signal separation is'acl complished by means of circuits' whichare.illustrated in" FIG. 1A.When subscribers talk over a telephonesystem, the voice signals comprisea continuous spectrum" of frequencies havingcharacteristi'cs as shownin" FIG;

In addition, th'ef 1B. Band-pass filters 1 and 4 are adapted to note thevoice signals by eliminating a narrow band at about 1000 cycles, forexample (FIG. 1C), thereby creating a discontinuous spectrum offrequencies. Thereafter, any suitable signalling equipment Stand 6provides control signals of about 1000 cycles which are passed bybandpass filters 2 and 3. The control signals fall within the notchprovided by band-pass filters 1 and 4 as indicated by the dotted-linecurve in FIG. lC. The troubles with this system are: first, the notch inthe voice frequency spectrum which is produced by filters 1 and 4 causesa degrading of the voice signal, and second, the signal frequencies andvoice frequencies tend to overlap, thus requiring expensive band-passfilters which have sharp cuto characteristics.

FIG. 1D illustrates the manner in which the subject invention providesfor signalling. A subscriber at station A has a signal sending device 7which applies voice frequency tones to the line in the sa-rne mannerthat voice signals #are applied. Therefore, the signals transmitted overline 9 include both voice yand control frequencies shown in' FIG. 1B.The voice spectrum is continuous; there is no degrading notch as shownin FIG. 1C. Although thevoice transmission is unimpaired, there is aproblem of detecting control signals while maintaining immunity torandom or spurious frequencies in the voice signal.

The subject invention samples or bleeds-off, through a limitingresistance 8, a sample of all signals passing between subscribers A andB via channel 9. The sample or lbled-off signal is then fed into asignal channel (FIG. 2) Where an vanalysis is made of the contentthereof. If a broad band of frequencies is present, it indicates thatsubscribers A #and B are conversing; therefore, frequencies that areused -as control signals are minimized so that the signalling equipmentmay not be influenced thereby. On the other hand, if there is no broadbrand of voice frequencies, the frequencies that are used as controlsignals are maximized whereby lthe signalling equipment of FIG. 2 may beactuated. Thus, the system of FIG. y1D separates the voice yand controlchannels by, in effect, continuously analyzing a sample signal divertedthrough resistance S to detect the presence of a Wide band of voicesignals. Preference is given to the wide band of voice signals ifpresent Iand preference is given to the narrow band of control signalsif the voice signals are not present.

Detailed Description Referring to FIG. 2, both voice and control signalsare transmitted over channel 9. A sample of all such signals is divertedor bled-olf through limiting resistance 8 and applied to suppressor 11which is adapted to distort the signals by amplifying la broad band offrequencies that do not fall within the range which is used fortransmitting control signals. Item 12 is an amplifier having an outputsignal which is constant despite a wide range of variations which mayoccur in the input signal. Item 13 comprises a plurality of tonedetection channels, each being tuned to lan individual frequencyidentified by the letters S, U, V, W, X, Y and Z. Control data istransmitted over channel 9 by signals consisting of one or more of theenumerated frequencies, e.g. the S frequency may be a seizure signal,frequencies V and W may represent the numeral 1, frequencies X and W mayrepresent the numeral 2, etc. Each of the channels 145-19 is providedwith tall of the components shown in the S" and U channels except thatnetworks similar to 51 and 54 are tuned to different frequencies.

Means is provided for improving voice immunity by changing Vtheamplitude distribution of incoming voice frequency signals. Moreparticularly, signal suppressor 11 (having characteristics as shown inFIG. 3A) comprises an amplifier 21 and a low pass lter 22 whichcooperate to amplify voice frequencies that rarevlower than the controlsignal band. The lamplification of suppressor 11 is relatively high inthe range-marked Voice Signal (FIG. 3A)-havring a frequency which islower than control signals yand the amplification is relatively low inthe range lof control signals.

Reference may be had to FIG. 3B for a graphical illustration of themanner in which signal suppressor 11 and constant output `amplifier 12cooperate to eliminate control signals during periods when a voicesignal ris on line 9. In FIG. 3B, the space between the lower linemarked 0 Volts and the upper line marked 2 Volts represents the totaloutput of amplifier 12. Quite obviously, these values are selectedmerely for purposes of illustrationany suitable signal values may beused. Curve I (FIG. 3B) indicates graphically the total output ofamplifier 12 under -all conditions. Curve II indicates the content ofthe output 'signal from amplier 12 which would occur When frequencies inboth the voice yand control range are present, if suppressor 11 were notprovided. For example, during telephone conversations, voice currentsmay include frequencies which fall Within lthe portion of the acousticspectrum that is used for transmitting control signals, ic. the upperportion of curve II. The remainder of the voice signal is indicated bythe lower portion of curve Il. Since suppressor 11 is present andfurther since the total output of lamplifier 12 does not change, theoriginal -signals (curve Il) are distorted as shown by curve III. Thatis, a broad baud of voice sign-als is amplified so that it occupies 'alarger portion of the output signal of amplifier 12 thereby reducing therelating signal strength of the control frequenoies. The conversingsubscribers are not aware of any distortion in their voice signals sincethe sounds which they hear result from signals that are transmitted overchannel 9 and are not effected by anything which happens in suppressor11. Insofar as the tone detectors of FIG. 2 are concerned, thedistortion caused by suppressor 11 is not objectionable since the objectis either to minimize or to maximize control signals depending uponthepresence or absence of Voice signals and 'further since suppressor 11has a flat response in the control signal range.

During periods when control signals lare to be effective, there is novoice signal on line 9. Signal suppressor 11 provides little or -noamplification. The total content of the input signal applied toamplifier 12 comprises tone components which are used for signallingpurposes. Since the output of `amplifier 12 does not change responsiveto rthe iiuotuations in the input signal, the total output ofamplifier12 consists of tone components which lare used for control purposestogether with any noise which may occur. Curve IV (FIG. 3B) illustratesthe output of amplifier 12 when only two tone componentstogether withnoise -are present. Tone vdetectors 13 are ladapted to respond tocontrol signals having a strength las indicated by curve IV but not torespond to control -signals having a strength as indicated by curve III.

Next reference is made to the amplifier of FIG. 4 which is used assuppressor 11 in FIG. 2. While any suitable electrical values may beaccommodated, it has lbeen found that signalling mlay be accomplished ina typical telephone system when all signals above 1.3 kc. are suppressedby about 7 db. Thus, the random frequencies falling in the control bandwhich occur during ordinary speech are suppressed by about 7 db and thesignal to noise natio is decreased by about 1.5 db due to the :gainwhich is introduced by the amplifier for frequencies of less than l kc.The amplifier response is substantially at in the control signal band.These characteristics result from the circuitry of FIG. 4 whereinresistance 8 limits fthe signal which is sampled or bled-off voicechannel 9. The emitter circuit determines the slope of the outputcharacteristics at low frequencies and the collector circuit provides alow pass filter which shapes the amsgroasssf plifiers' outputcharacteristics at higher frequencies as shown inFIG. 3A.

FIG. illustrates the circuitry 'which is* used to complete thehollovvfboxes shownin-the tone detector for frequency S (FIG. 2) andlwhichis duplicated ineach ofthe other tone detector channels. Itemf 51is atuned network which is adapted to pass frequencies havingcharacteristics as shown in- FIG. 6. The output off/tuned' network 51-may fall any place 'between curve A and curveA B (FIG. 6i) due to thepossible variations of a compound signal'tone, i.e-.V there may be (l)rio-noise on line 9" and both tone componentsy may have' equalamplitude, rand (2)..noise on line 9. and the two tone components' mayhave unequal amplitude.- Since the detector channel must respond to theminimum voltage at which-control signals may occur, the circuit must 4bearranged to respond to signals having a minimum voltage which isindicated by point C. While curve B has `the proper bandwidth at'the.voltage that is indicatedby the letter C, curve A lias a much' broaderbandwidth at the voltage of point C. Therefore, alirnited bandwidth ofcontrol frequencies is selectedby alternate tuned networks and limiterstages.

Signals. from constant'. output` amplifier 12 (FIG. 2) are applied tothe terminal marked IN (FIG. 5). As indicated in FIG. 5, a' transformersuch. as item 502 is provided for eachvv signal channel, for. example,trans-r former 502A may be in channel 1'8v and transformer 502B maybe inchannel 19 with other transformers (notsliown) provided for eachadditional' channel. The primary 'winding of'each transformer islconnectedy in series and driven .by a common amplifier `21: .'Ilhesecondary'winding oif each transfonneris connected to individual channelequipment. 'Resistances such as 501- and capacitances such as 503cooperate to tune each channel tol the desired frequency and further toprovide a bandwidth which is less than the frequency difference betweenadjacent channels, thereby preventing any subharnonic from passing thefollowing limiter.

'Dhe output of tuned transformer 502 isapplied to transistorizedhalf-wave amplifier 50'4 which preventsloading of tune'dcircuit 51.Resistance 505- is provided to control the input impedance of emitterfollower 54M. 'Resistance 506 tends to keep the load -reected from thelimiter of amplifier 504 constant even at higher voltages.

Ampliiier 507 limits the signal'l as' shown by curve- D (FIG. 6`) to thevoltage of point C, i.e. limiter 507 provides an output signal havingconstant strength regardless of whether tuned circuit 51 passes sign-alsat levels indicated by curve A (FIG. 6) or by curve B or at someintermediate point above level Cin curve B. Capacitance 508i is adecoupling device which passes A.C. signals to ground. Resistances 509and 5-11 provide a voltage :divider 'which determines the emitter biasof limiter 507. Resistance 510 stabilizes the lgain of transistor '507,i.e. the sensitivity of the limiter. 'Resistance 51-2 lowers thecollector voltage to provide a better limiting laat-ion.

Items 513 and 514 constitute a tuned circuit which has outputcharacteristics as shown by curve E in FIG. 6. Since the originallyapplied input signal has been limited as shown by Icurve D', tunedcircuit 54 passes only the required bandwidth at voltages indicated bypoint F, thereby eliminating the problems created by widely fluctuatingsignal strength as indicated by curves A and B.

Detector 55 is in effect, a rectifier having a high input impedance anda low output impedance providing a D.C. output voltage which isproportional to the A.C. voltage across tuned circuit 54. -Resistor '516limits the current and 'capacitor 517 acts as charging capacitor.

'Capacitor 518, resistance 519' and diode 520' provide a time delaycircuit Ihaving relatively long charge but short discharge time which isadapted to limit the response of the tone detector to signals having atleast a predetermined duration. In this manner, momentary voice signalsVfalling in the control band may not -trip the tone detector. In greaterdetail, the output of ytransistor 515 is fed through 6. resistance yS19to charge capacitance 518over an extended periodA of Diode 520. is poledso that it does not pass charging current.V Afterrlcapacitor 51$ hascharged sufficiently, transistor 521 begins toA conduct. The` dischargeof capacitor 51 Sisvery quicksince diod'efSZl)t is poled .to provide alowresistance circuit to: pass dischargingzcurrent through resistance516ml ground. This is very important because it preventscharlgingof'capacitorflSY-frorn short signal-frequency pulses containedv inspeech which might otherwise accumulate-over a-period' of time.Transistors 'S21 and'5l22l act as pulse amplifiers Iwith acharacteristicas shown by curve vG (FIG. 6); Transistorfizl'isza power output stage.Thel outputr signal atiterminalSMf may be fed into logic circuitry toprovide anyi suitable control function.- Resistance 5126i is sensitivetotemperaturevariations, thereby automatically adiustingthe' gain oftransistor 5211 to compensate for any. temperature variations in thedetector channel.

While the principles y of the invention have been described aboveconnectionwith specific apparatus, it is to be clearly understood that.this description'v is made only by way of'example and not as alimitation to the scope of ther invention.

' of awide band of signals, said analyzing meanscomprising a firstamplifier coupled` to amplify at least a portion of` said wide band"`ofsignals andv to'. have sub-` stantially no'` amplifying effect on saidlimited band, a constant output'amplfifier coupledr to' amplify theoutput of the said first amplifier, meansv including; said constantoutput amplifier and renderedV effective responsive to saidamplification' ofsaid portion of`said wide band of signals for renderingsaid control signals: ineffective, and meansr including saidi constant.'output amplifier and renderedA effective.'` responsive to an absence ofsaid arn-k plified portion of said wide band ofsignals for.' renderingsaid" control signals effective: f

2. The chann'ei'of'claim l` and meansincluding:alter-l nate' tunedcircuits and' limiting.' circuits for. selectingrsaid limited bandwidth,means for detecting signals in said selected band, means for measuring apredetermined period of time following said detection of said selectedsignals, and means effective at the end of said predetermined timeperiod for causing a circuit response if said detected signal continues.

3. The detector of claim 2 wherein said means for measuring apredetermined period of time comprises means including a capacitorhaving a relatively long charging time, and a relatively shortdischarging time.

4. A signal separation system comprising mea-ns for conveying aparticular spectrum of signals including a plurality of different typesof signals falling within said spectrum, means for separating saidsignal-s according to lsaid types, said separating means comprisingmeans for distorting the amplitude distribution of said signals todecrease the relative strength of one type of said signals in thepresence of another type of said signals, and to increase the relativestrength of said onetype of said signals in the absence of said othertype of said signals wherein said distorting means comprises acombination of a first amplifier :tuned to amplify only said other typeof said signals, a second amplifier coupled thereto to provide an outputsignal of constant signal strength regardless of any fluctuations ofinput signal strength, whereby substantially the total output of saidsecond amplifier comprises said other type of signals when present andcomprises said one type of signals when said other type of signals arenot present, means for responding to said one type of said signals whendistorted to have relatively high signal strength, and means forrejecting said one type of said rsignals when distorted to have arelatively weak signal strength.

5. A signal separation system comprising means for conveying aparticular spectrum of signals including a plurality of different typesof signals falling within said spectrum, means for separating saidsignals according to said types, said separating means comprising meansfor distorting the amplitude distribution of said signals to decreasethe relative strength of one type of said signals in the presence ofanother -type of said signals and to increase the relative strength ofsaid one type of said signals in the absence of said other type of saidsignals, wherein said one type of said signals comprises frequenciestransmitted in a relatively narrow bandwidth, said other type of saidsignals comprises frequencies transmitted in a relatively widebandwidth, said narrow bandwidth falling within said wide bandwidth,said distorting means comprises an amplifier having characteristics suchthat at least a portion of said wide band is amplified greatly comparedto the said narrow band, means for responding to said one type of saidsignals when distorted to have relatively high signal strength, andmeans for rejecting said one `type of said signals when distorted tohave a relatively weak signal strength.

6. The system of claim and means comprising a constant output amplifiercouplied to receive and amplify the output of said distortion producingamplifier whereby substantially all of the signal of said constantoutput ampliiier comprises said wide band of signals when present andsaid narrow band of signals when said wide band of signals Iis notpresent.

7. In the system of claim 5 tone detector means comprising meansincluding a plurality of circuits for passing frequencies falling insaid narrow bandwidth, said circuits being alternately tuned circuitsand limiting circuits, means for detecting signals passed through saidalternately tuned and limiting circuits, means for measuring apredetermined period of time following said detection of said signals,and means effective at the end of said measured time period for causinga response if said passed signals are still present.

i8. A telephone system comprising a voice channel, means fortransmitting a continuous spectrum Lof voice current signals throughsaid channel, means for transmitting through said channel a relativelynarrow bandwidth of control signals falling within said spectrum, meansfor separating said voice signals and said control signals comprisingmeans for continuously sampling the current being transmitted through-said channel, means for amplifying all voice current signals in saidsample having a frequency which is less than said narrow band, meanscoupled to said last named means for maintaining a constant signalstrength regardless of fluctuations in the output s-ignal of said lastnamed means, means comprising a network tuned to pass said narrowbandwidth connected to said constant signal means, means coupled to saidtuned inetwork for limiting the signal strength of current passedthrough said tuned network, and means comprising'a second network tunedto pass said narrow bandwidth connected to transmit the signals passedthrough said limiting means.

9. The system of claim 8 and detector means for rectifying signalspassed through said second tuned network, and lter means connected tosaid detector means to provide a relatively smooth direct currentresponsive to said rectied signals'.

lO. The system of claim 9 and means comprising a resistance andcapacitance for measuring a predetermined period of time during whichsaid capacitance charges responsive to flow of said relatively smoothdirect current, and means comprising a diode coupled across saidresistance and poled to provide a very fast discharge time constant forsaid capacitance.

11. The system of claim 10 and means coupled to transmit an eifectiveoutput signal after a period of time measured by said charging of saidcapacitance if said direct current continues.

References Cited in the tile of this patent UNITED STATES PATENTS2,364,685 Baker Dec. 12, 1944 2,577,614 Fnitschi et al. Dec. 4, 1951.2,698,878 Martens Ian. 4, 1955 2,806,903 Hargreaves Sept. 17, 19572,830,128 Radcliff et al. Apr. "8, 1958 2,883,474 Fritschi Apr. 21, 19592,929,880 Koehler Mar. 22, 1960 2,935,572 Hastings et al. May 3, 19602,964,650 Radcliff et al. Dec. 13, 1960

5. A SIGNAL SEPARATION SYSTEM COMPRISING MEANS FOR CONVEYING A PARTICULAR SPECTRUM OF SIGNALS INCLUDING A PLURALITY OF DIFFERENT TYPES OF SIGNALS FALLING WITHIN SAID SPECTRUM, MEANS FOR SEPARATING SAID SIGNALS ACCORDING TO SAID TYPES, SAID SEPARATING MEANS COMPRISING MEANS FOR DISTORTING THE AMPLITUDE DISTRIBUTION OF SAID SIGNALS TO DECREASE THE RELATIVE STRENGTH OF ONE TYPE OF SAID SIGNALS IN THE PRESENCE OF ANOTHER TYPE OF SAID SIGNALS AND TO INCREASE THE RELATIVE STRENGTH OF SAID ONE TYPE OF SAID SIGNALS IN THE ABSENCE OF SAID OTHER TYPE OF SAID SIGNALS, WHEREIN SAID ONE TYPE OF SAID SIGNALS COMPRISES FREQUENCIES TRANSMITTED IN A RELATIVELY NARROW BANDWIDTH, SAID OTHER TYPE OF SAID SIGNALS COMPRISES FREQUENCIES TRANSMITTED IN A RELATIVELY WIDE BANDWIDTH, SAID NARROW BANDWIDTH FALLING WITHIN SAID WIDE BANDWIDTH, SAID DISTORTING MEANS COMPRISES AN AMPLIFIER HAVING CHARACTERISTICS SUCH THAT AT LEAST A PORTION OF SAID WIDE BAND IS AMPLIFIED GREATLY COMPARED TO THE SAID NARROW BAND, MEANS FOR RESPONDING TO SAID ONE TYPE OF SAID SIGNALS WHEN DISTORTED TO HAVE RELATIVELY HIGH SIGNAL STRENGTH, AND MEANS FOR REJECTING SAID ONE TYPE OF SAID SIGNALS WHEN DISTORTED TO HAVE A RELATIVELY WEAK SIGNAL STRENGTH. 